1. Field of the Invention
The present invention relates to a spin valve sensor in a read head that has an encapsulated keeper layer and more particularly to a keeper layer that is encapsulated by top, bottom, first side and second side insulation oxide layers and to a method of making.
2. Description of the Related Art
A high performance read head typically employs a spin valve sensor for sensing signal fields from a magnetic medium, such as a rotating magnetic disk or a linearly moving magnetic tape. The sensor includes a nonmagnetic electrically high-conductance spacer layer sandwiched between a ferromagnetic reference layer, which is sometimes referred to as a pinned layer, and a ferromagnetic sense layer, which is sometimes referred to as a free layer. An antiferromagnetic pinning layer interfaces the reference layer for pinning the magnetic moment of the reference layer in a direction transverse to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the magnetic medium. First and second hard bias and lead layers are connected to the spin valve sensor for conducting a sense current therethrough. The magnetic moment of the sense layer is free to rotate in positive and negative directions from a quiescent or bias point position in response to positive and negative signal fields from the moving magnetic medium. The quiescent position is the position of the magnetic moment of the sense layer when the sense current is conducted through the sensor without signal fields from the moving magnetic medium. The quiescent position of the magnetic moment of the sense layer is preferably parallel to the ABS. If the quiescent position of the magnetic moment of the sense layer is not parallel to the ABS, the positive and negative responses of the sense layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered by the interfaces of the spacer layer with the sense and reference layers. When the magnetic moments of the sense and reference layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximal. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. A change in resistance of the spin valve sensor is a function of cos .theta., where .theta. is the angle between the magnetic moments of the sense and reference layers. The maximum change determines a giant magnetoresistance (GMR) coefficient .DELTA.R.sub.G /R, where .DELTA.R.sub.G is the change in resistance of the spin valve sensor from minimum resistance where the magnetic moments of sense and reference layers are parallel to maximum resistance where the magnetic moments of the sense and reference layers are antiparallel, and R is the resistance of the spin valve sensor at minimum resistance. For this reason it is sometimes referred to as a GMR sensor.
The transfer curve (magnetoresistance or readback signal of the spin valve head versus signal fields from the moving magnetic medium) of a spin valve sensor is a substantially linear portion of the aforementioned function of cos .theta.. The greater this angle, the greater the resistance of the spin valve sensor to the sense current and the greater the readback signal (voltage sensed by the processing circuitry). With positive and negative signal fields from the moving magnetic medium (assumed to be equal in magnitude), it is important that positive and negative changes of the resistance of the spin valve sensor be equal in order that the positive and negative magnitudes of the readback signals are equal. When this occurs a bias point on the transfer curve is considered to be zero and is located midway between the maximum positive and negative readback signals. When the direction of the magnetic moment of the sense layer is parallel to the ABS, and the direction of the magnetic moment of the reference layer is perpendicular to the ABS in a quiescent state, the bias point is located at zero and the positive and negative readback signals will be equal when sensing positive and negative signal fields from the moving magnetic medium. The readback signals are then referred to in the art as having symmetry about the zero bias point. When the readback signals are not equal the readback signals are asymmetric.
The location of the bias point on the transfer curve is influenced by three major forces on the sense layer, namely a ferromagnetic coupling field (H.sub.F) between the reference layer and the sense layer, a demagnetizing field (H.sub.D) from the reference layer, a sense current-induced field (H.sub.I) from all conductive layers of the spin valve sensor except the sense layer.
In the spin valve sensor, the demagnetizing field from the reference layer rotates the magnetic moment of the sense layer toward a first direction perpendicular to the ABS. This demagnetizing field is counteracted by a ferromagnetic coupling field H.sub.F of the reference layer and a sense current-induced field that rotate the magnetic moment of the sense layer toward a second direction antiparallel to the first direction. The sense current-induced field is imposed on the sense layer by the pinning layer (if conductive), the reference layer and the spacer layer, which are all on one side of the sense layer.
In order to minimize the sensitivity of readback signal to the sensor stripe height, the demagnetizing field from the reference layer must be reduced. This can be achieved by providing a ferromagnetic keeper layer on an opposite side of the sense layer from the reference layer with a nonmagnetic electrically low-conductance spacer layer between the sense layer and the keeper layer. With this arrangement the keeper layer provides a flux path for the demagnetizing field from the reference layer and, in turn, the reference layer provides a flux path for the demagnetizing field from the keeper layer. Consequently, the reference and keeper layers provide a nearly closed loop for the demagnetizing fields from both of these layers so that only a small demagnetizing field, resulting from a net magnetic moment between the reference and keeper layers, are imposed on the sense layer to influence its bias point. It is important that the magnetic moment of the keeper layer be oriented antiparallel to the magnetic moment of the reference layer. This can be assured by directing the sense current in a proper direction to the spin valve sensor so that sense current-induced fields urge the magnetic moment of the keeper layer to be antiparallel to the magnetic moment of the reference layer. An additional benefit of the keeper layer is that it exerts a sense current field on the sense layer that is in an opposite direction to the direction of the sense current-induced fields from the pinning layer (if it is conductive), the high-conductance spacer layer and the reference layer, in the sense layer. As a result, both the demagnetizing and sense current-induced fields in the sense layer of the keepered spin valve are much smaller than those in the sense layer of the basic spin valve. In particular, a small demagnetizing field in the sense layer leads to the minimization of the sensitivity of readback signal to the sensor stripe height.
However, a keeper layer shunts a portion of the sense current which reduces the GMR coefficient (.DELTA.R.sub.G /R) of the spin valve sensor. A reduction in .DELTA.R.sub.G /R causes difficulties in miniaturizing the spin valve sensor while still maintaining high readback signals. A high .DELTA.R.sub.G /R thus must be maintained for the miniaturized spin valve sensor to exhibit high readback signals and a higher bit density (bits/square inch of the rotating magnetic disk). Efforts over the years have increased the storage capacity of computers from kilobytes to megabytes to gigabytes. The use of a keeper layer defeats these efforts. There is a strong felt need of a keeper layer that does not shunt a portion of the sense current so that bit density can be increased. Ease in a control of read signal asymmetry is also an important factor in increasing bit density.